1. Field of Invention
The present invention relates to a charger control circuit and a charger control method.
2. Description of Related Art
One example of the chargers is the photoflash capacitor charger. Such charger has a basic structure as shown in the upper half of FIG. 1, in which an input terminal Vin charges a capacitor Cout at the output terminal Vout through a transformer 10. The charging time is controlled by a power switch 21 in a charger control circuit 20. The charger control circuit 20 is usually an integrated circuit (IC). In the prior art shown in FIG. 1, the operation of the power switch 21 is controlled by a switch control circuit 25, which determines the ON time of the power switch 21 according to a detection result obtained from the secondary side of the transformer 10. More specifically, the resistor R in the circuit converts the secondary current to a voltage signal, and this voltage signal is compared with a reference voltage Vref in a comparator 23. When the voltage signal is lower than the reference voltage Vref, it means that the secondary current is close to zero and there is no current charging the capacitor Cout. Hence, the switch control circuit 25 turns ON the power switch 21 so that an induced current is generated by the transformer 10 to charge the capacitor Cout. The OFF time of the power switch 21 is determined by the primary current (relevant circuit not shown); when the primary current reaches a predetermined threshold, the switch control circuit 25 turns OFF the power switch 21.
The above mentioned prior art has the drawback that a large amount of current will flow into the charger control circuit 20, causing noises and errors. In addition, large primary current spikes would occur.
FIG. 2 shows another prior art circuit which senses the charging current by obtaining a signal from the primary side. More specifically, it compares the voltages at the two sides of the primary winding, and the switch control circuit 25 controls the ON time of the power switch 21 according to the comparison result. When the difference between the voltages at the two sides of the primary winding is close to zero, it means that there is no induced current flowing on the transformer 10, and hence the switch control circuit 25 turns ON the power switch 21. This prior art solves some of the drawbacks of the prior art in FIG. 1; however, in both the prior art of FIGS. 1 and 2, the charger control circuit IC 20 requires two pins (P1 and P2, or P1 and P3) to detect the charging current for feedback control.
In view of the foregoing drawbacks, it is desired to provide a charger control circuit with less number of pins, and it does not require obtaining a feedback signal from the secondary side.